1. The Field of the Invention
Embodiments of the present invention relate generally to electron emitters and their methods of assembly. More particularly, disclosed embodiments are directed to electron emitter assemblies suitable for thermionic emission of electrons for x-ray generation.
2. The Relevant Technology
The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
An x-ray tube typically includes a vacuum enclosure that contains a cathode assembly and an anode assembly. The vacuum enclosure may be composed of metal such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. At least a portion of the outer housing may be covered with a shielding layer (composed of, for example, lead or a similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition, a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating it to an external heat exchanger via a pump and fluid conduits. The cathode assembly generally consists of a metallic cathode head assembly and a source of electrons highly energized for generating x-rays. The anode assembly, which is generally manufactured from a refractory metal such as tungsten, includes a target surface that is oriented to receive electrons emitted by the cathode assembly.
During operation of the x-ray tube, the cathode is charged with a heating current that causes electrons to “boil” off the electron source by the process of thermionic emission. An electric potential on the order of about 4 kV to over about 200 kV is applied between the cathode and the anode in order to accelerate electrons boiled off the electron source toward the target surface of the anode assembly. X-rays are generated when the highly accelerated electrons strike the target anode surface.
Most of the electrons that strike the anode dissipate their energy in the form of heat. Some electrons, however, interact with the atoms that make up the target and generate x-rays. The wavelength of the x-rays produced depends in large part on the type of material used to form the anode surface. X-rays are generally produced on the anode surface through two separate phenomena. In the first, the electrons that strike the anode surface carry sufficient energy to “excite” or eject electrons from the inner orbitals of the atoms that make up the target. When these excited electrons return to their ground state, they give up the excitation energy in the form of x-rays with a characteristic wavelength. In the second process, some of the electrons from the cathode interact with the atoms of the target element such that the electrons are decelerated around them. These decelerating interactions are converted into x-rays by conservation of momentum through a process called bremsstrahlung. Some of the x-rays that are produced by these processes ultimately exit the x-ray tube through a window of the x-ray tube, and interact with a patient, a material sample, or another object.
Generating a tightly collimated x-ray beam for diagnostic purposes can be achieved by maximizing both x-ray flux (i.e., the number of x-ray photons emitted per unit time) and the focusing of the electron stream on the anode surface in order to produce a tightly collimated x-ray beam. Diagnostic image quality is at least partially a function of the number of electrons that impinge upon the target surface of the target anode. In general, more electrons results in higher x-ray flux, which in turn results in x-ray images with higher contrast (i.e., higher quality). In addition to emitter efficiency, the quality of diagnostic images can additionally depend on the pattern, or focal spot, created by the emitted beam of electrons on the target surface of the target anode. In general, a smaller focal spot produces a more highly focused or collimated beam of x-rays, which in turn produces better quality x-ray images.
In conventional x-ray tube designs, it is often difficult to achieve an optimal electron beam emission, due to a number of design constraints and tradeoffs. For example, emitter assemblies that utilize a planar emitter structure are desirable because such emitters are more useful for shaping the electron beam and consequent focal spot on the anode. However, planar emitters, due to their structure, are difficult to mount within a device due to the extremely high temperatures needed for thermionic emission. Such temperatures often exceed the capability of materials used in the planar emitter, and also lead to relatively large thermal expansion of the emitter assembly. Such thermal expansion often results in relative high variability in the resulting focal spots, which decreases the optical precision and ratability of the x-ray tube.